Existing Doppler-based ultrasonic haemodynamic monitors require significant skill and experience on the part of the operator to set the signal gain in the signal amplifier, to thereby ensure a suitable signal is presented to the Fast Fourier Transform (FFT) analyser for analysis. Even when a suitable signal is presented, it is still possible for the operator to significantly influence the output data derived from the machine, by varying the gain. Further potential for confusion arises from the fact that these forms of apparatus require the operator interpreting the displayed waveform and other output data, to be able to distinguish that a change in displayed data is a result of transducer movement rather than a change in the patient's cardiac function.
For apparatus of this type to be truly useful in a clinical environment, it is important that consistent output data is produced when a patient is being monitored, and that the quality of the data is minimally dependent on the skill of the operator in setting up the machine. If this is achieved then clinicians can determine and publish a range of ‘normal’ or ‘acceptable’ data for all patients, thus aiding the process of diagnosis.
Attempts have been made, in the past, to produce an ‘automatic’ system for establishing gain. These have not been successful, however, as they have tended to utilise perceived signal strength with no regard to the point in the pulsitile flow at which this signal strength was detected. These known systems have also incorporated continuous adjustment of the gain rather than determining a base value and then fixing the gain at that value. This approach causes particular problems in haemodynamic monitoring because, as stated above, movement of the flow transducer with respect to the flow being monitored can result in incorrect data being presented to the operator. A continuous automatic gain system has been found to mask probe movement by increasing the gain as signal strength declines with probe movement when, of course, the operator should have been informed that the transducer appeared to be moving out of alignment with the flow.
Prior art systems do exist which inform the operator if movement of the transducer is detected, but these systems typically incorporate a further transducer and associated electronics to determine alignment of the transducer with the vessel through which flow is being monitored.
It is therefore an object of this invention to provide a method for automatically setting gain, and apparatus which incorporates such a gain setting facility, which goes at least some way in addressing the drawbacks identified above; or which will at least provide a useful choice.